A photonic crystal which is an optical material having a refractive index distribution in almost the same period as the wavelength of light beam has gotten a lot of attention as a new optical material that can freely control the propagation and the generation of light beams. In the photonic crystal, a fine grating is formed on the surface of the material with the grating period in a nanometer-order. The fine grating is conventionally formed by using thin film techniques (semiconductor process) such as an X-ray lithography or an electron beam lithography.
However, in order to perform the thin film technique, expensive facilities such as a film forming equipment, an exposure equipment, a lithography equipment, and an etching equipment are required. In addition, since a long time is required for processing, mass production can not be performed.
On the other hand, in recent years, an inexpensive processing with a high degree of efficiency has become possible by the appearance of an ultra-precision working machine. There are various types of working methods used in the ultra precision working machine such as fly cutting, planer, end mill, and lathe working methods.
The ultra-precision working machine can form gratings with the grating period of 1 μm and more with a sufficient degree of accuracy. However, in the field of nano-technologies such as a photonic crystal, it is naturally demanded that the grating period should be 1 μm or less. When the ultra-precision working machine performs in such a nanometer-order, a burr is formed and the roughness and the swelling of a worked surface are increased. On the contrary, a long working time is required when a deburring is performed after cutting work.
In an optical element used in various types of optical devices, a multi-layer film for anti-reflection is often formed on the surface of the optical element to decrease the energy loss and the stray light due to the reflection of a light beam. In such a multi-layer film, the reflection is prevented in such a manner that the respective reflected lights between the respective layers cancel each other through the interference.
However, when the multi-layer film is used for anti-reflection, the material having an optimal refractive index as the multi-layer film is limited. Thus a combination of optimal materials may not exist depending on a blank material and the wavelength of the incident light. Furthermore, the heat resisting properties and the durability may be reduced due to the physical, chemical and thermal inconsistency based on the differences of the materials of the respective layers. In addition, since the multi-layer film is formed by means of the vapor deposition, the cost of material increases.
Instead of the multi-layer film, another anti-reflection method has been known in which a grating comprising of minute projections with a shorter period than the wavelength of light is constructed on the surface of an optical element. The grating comprising minute projections with the period shorter than the wavelength of light is equivalent to a medium having a certain refractive index and gives the same anti-reflection effect as the multi-layer film. In addition, when the minute projection is formed in a conical shape or pyramid shape, its volume occupancy rate gradually changes from the base portion of the minute projection to the front end side thereof. Therefore, since the effective refractive index on the surface gradually varies, the anti-reflection characteristics are improved. Alternatively, the construction in which projections and recesses are formed on the surface at random has been proposed to realize similar anti-reflection characteristics (for example, Japanese Patent Laid-Open No. 2002-286906). However, when the grating comprising many minute projections as described above is formed by mechanical processing, it is very difficult to easily manufacture a grating such that a burr is not formed.